well yeah, basically cold solder is where the solder joint starts to crack. there have been several instances, even from my own personal experience, where it just happens that things are bent a certain way and make decent enough contact that you'd never notice it. then once you cap a monitor for instance, you start to notice all kinds of strange irregularities.
it happened when I did my two PlayChoice-10 monitors in fact. they were perfectly fine until I capped them. then all of a sudden I had primary colors missing, or you could tap on the cab and they'll jitter and shit. same thing with my Donkey Kong. part of my required things to do when I cap Nintendo monitors now is I reflow EVERYTHING on the neckboard.
and of course, cold solder has to strike sometime, right? lol
considering the age of these Polo units, they're approaching the 20 year mark now. I actually have one in my KI that's MINT and even it had certain issues with cold solder. just saying.
I still maintain the theory that whatever affected components in your monitor were just positioned a certain way and they may have moved. I know you say you didn't move the cab, but power cycles with heating and cooling of components can cause them to move just enough to cause what appears to be poor sync in your case. I'm surprised you get a legible enough image on yours though, usually if there's NO sync you'll just have a jumble of squiggly lines going diagonally.